This invention relates to pilot operated relief valves that provide either maximum pressure limiting or provide pressure regulation under varying flow rates and over a selectable range of pressures. An inherent property of the present invention relates it further to pilot actuated on-off valves.
Pilot operated, or as it is frequently called, "two stage valves" are employed where pressure regulation above approximately 1000 psi is required. Single stage valves, except in very small sizes, cannot operate in this high pressure range primarily because of the limit imposed by loading spring stress. Very stiff springs, which could provide the loading force, cannot be used because of the very poor pressure regulation that results. In pilot operated, pressure regulating valves, a controlled pressure is employed to provide substantially all of the loading force; and consequently, the loading spring can be lightly stressed even at very high pressures. In such valves of the prior art, the loading pressure is provided by an orifice fed, pilot relief valve that is in parallel with the pressure regulating valve. The loading spring stress of the pilot relief valve limits the maximum pressure to approximately 5000 psi. The present invention is directed toward means for raising the maximum pressure and for eliminating the clogging, silting, stability, pressure range and droop difficulties associated with highly spring loaded pilot relief valves. The properties of this invention also permit the use of relatively low force pilot relief valve springs and actuators to control the pressure setting, even at the highest pressure.
Solenoid actuated on-off valves in current use employ a spring biased first stage valve and a second stage pilot circuit that is the same as emp1oyed in the two stage pressure regulating valve described above, with the exception that the second stage valve is not configured for flow area modulation but for switching between maximum flow area and zero. At maximum flow area the first stage valve is driven open by system pressure; and at zero flow area the first stage valve is held closed by the spring bias and the area unbalance that results from the first stage seating shoulder. If the pilot valve of this circuit were configured to provide means for setting fixed orifice areas the first stage valve would reach an equlibrium position at a system pressure that corresponds to the set orifice area, the system pressure increasing as the orifice area is set to smaller values. This pressure regulating system eliminates the problems and limitations of the the spring biased pilot valve; but, it is not in use in pressure regulating systems because the droop in regulation is excessive.